Solid acids prepared by adding sulfuric acid on silica gel have been used as catalysts in the nitration of nitrobenzenes and their properties have been tested by kinetic studies at 25 degrees C. Nitration rates in concentrated aqueous solutions of sulfuric acid were also analysed and the catalytic efficiencies of sulfuric acid in liquid and solid phase were compared by using kinetic data of analogous compounds. The results show that the solid acid samples exhibit nitrating properties very similar to those observed in concentrated aqueous solutions of sulfuric acid (range of 90 wt%). The relationship between nitration rates and effective concentration of electrophilic species [NO2+], determined by studying the protonation-dehydration equilibrium of nitric acid in strong acids (HNO3 +H+ reversible arrow H2O +NO2+), was tested to better understand the acidity properties of medium.